Hox genes are metazoan pattern forming genes—genes that are universally associated with defining the identities of regions of the body. There are multiple Hox genes present, and one of their unusual properties is that they are clustered and expressed colinearly. That is, they are found in ordered groups on the chromosome, and that the gene on one end is typically turned on first and expressed at the head end of the embryo, the next gene in order is turned on slightly later and expressed further back, and so on in sequence. That the tidy sequential order on the chromosome is associated with an equally tidy spatial and temporal pattern of expression in the body has always been one of the more fascinating aspects of these genes, and they are one of the few cases where we see an echo of phenotypic form comprehensibly laid out in the DNA.
However, there are some exceptions to the tidy clustering, and they occur right in two animals that have been central to developmental/genetic research, Caenorhabditis elegans and Drosophila melanogaster. These animals have broken clusters. Almost everywhere else, the Hox genes are ordered in one place, but in two of the most common research organisms, they've been split apart into two groups…so what's going on? We have what looks a little bit like a universal rule in genetic organization, and then it gets violated with seemingly little consequence. How do worms and flies get away with it?
One way to find out is to look for more exceptions to Hox ordering, and here's a doozy: an animal, the tunicate Oikopleura dioica, has blown its Hox gene clusters to flinders and scattered the individual Hox genes all over its genome, with no detectable linkage between them.
Continue reading "Hox cluster disintegration" (on Pharyngula)